1. Field of the Invention
This invention relates to an enclosure assembly for use on elongate cylindrical objects.
More specifically, the invention is directed to sheathing sleeves intended for application over cable splices, or cable ends, in medium- to high-voltage power lines where the high electric potentials involved, being on the order of tens or hundreds kV in magnitude, require that air inclusions be totally absent from between the sleeve inner surface and the outer surface of a cable to be sheathed.
It is understood, however, that the enclosure assembly of this invention may be also employed for applications other than the above-outlined one, for instance over cylindrical objects of various description--such as pipes, rods, stems, and the like--e.g. to provide local protection against corrosion or else.
2. Description of the Related Art
As is known, in splicing electric cables together, it is common practice to remove locally protective and insulating members from around the conductor of either cable, at the ends of the cables to be spliced together, for connection to the conductor of the other cable.
After filling with appropriate materials (jointing compounds, self-amalgamating tapes, or the like) the gaps left by the removal of insulative and/or protective members, the spliced bare ends of the two cables are covered with a sheathing sleeve of an elastomer material which provides the necessary protection and insulation for proper performance of the splice.
The elastomer sleeve is preliminarly fitted, in a condition of elastic expansion, over a tubular cylinder body made of a rigid plastics material to be engaged around either cable prior to splicing.
Subsequently, after the conductors have been spliced to each other, the tubular body is removed from the elastomer sleeve, thereby allowing the sleeve to shrink elastically about the cables at the splice.
In a prior device, disclosed in Italian Patent Application 28578 A/88 and European Patent Application EP 89120585.8, the tubular cylinder body is removed by slipping it axially off the elastomer sleeve with the aid of appropriate tooling.
Another prior art device, as disclosed in Patent GB 1,292,608, provides a support consisting of a tubular cylinder body whereover an elastic covering is fitted. The tubular body is formed with a helical groove around its outer surface. In this way, a continuous strip is substantially created on the tubular cylinder body in the form of a plurality of turns laid consecutively and being set close against one another along a weakening line of reduced thickness which is defined at the bottom of the helical groove.
By exerting a pull force on a free end portion of the strip which extends from one of the turns at the tubular body ends and longitudinally across the body, the turns are progressively separated along said weakening line. Thus, the tubular cylinder body can be removed from the elastic covering by a simple manual operation requiring no special puller.
U.S. Pat. No. 4,503,105 provides a helical groove cut through the thickness of the tubular body. The individual turns are interconnected by longitudinal bridge formations on the tubular body interior whose thickness is only partly affected by the helical groove depth.
European Patent EP 0 291 203 discloses instead a tubular body wherein a plurality of through-going cutouts, set closely apart in a helical pattern, are formed using a suitable tool operated on the outer surface of the cylindrical body.
A further known support, shown in U.S. Pat. No. 4,389,440, has the tubular cylinder body formed by a band-like element which is wound helically into a plurality of side-by-side turns bonded to one another. Here again, by exerting a pull force on an extended portion of the band-like element laid longitudinally across the tubular body, the tubular body is caused to collapse as the turns come progressively apart.
It has been found that, for high values (200-300%) of radial deformation of the sleeve fitted over the support, axial deformations appear in the sleeve upon release thereof; that is, as the support is being removed, the sleeve not only shrinks elastically in the radial direction but also changes in length along the longitudinal direction.
The extent of said longitudinal deformation varies in a not easily foreseeable fashion, it being dependent on the materials utilized, the amount of radial deformation imparted, the amount of radial shrinkage of the sleeve--tied, in turn, to the size of the cables fitted with the sleeve, the way the sleeve is expanded on the support, the length of time that the expanded sleeve has been kept in store, etc.
The foregoing considerations bring out the difficulty of ensuring proper positioning of the elastic sleeve over the cable junction area, especially where particular dimensional limitations and tolerances exist, or in the presence of elements to be placed at the locations of some particular members of the spliced cables, internally of the sleeve.
Specifically, in one embodiment, the sleeves may be provided with an internal electric field deflector, or electrode, consisting of an annular portion of a semiconductive elastomer material; such an element, which is to be given specified shape and dimensions for the application on hand, should locate precisely in the central region of the splice, at the bare conductor portions connected by the clamp, such that said region can be shielded electrically, and the electric field nullified to make the presence of air and clamp irregularities ("peak" effects) ineffectual.
Improper positioning of the deflector relatively to the clamp would thwart its action and may develop electric field concentrations apt to result in the splice being pierced through.
In an alternative embodiment, in lieu of the deflector, the sleeve may have a layer of a field-control material; however, that embodiment also requires that the sleeve be accurately positioned especially as regards the ends of said field-control layer.
On the other hand, it is quite impractical to rely on the operator's skill for the solution of such problems, because the operator would be substantially unable to alter the position of the deflector or the sleeve at the installation stage of the sleeve on the cables, or to change it after shrinking.
In accordance with this invention, it has been found that a possibility of control lies in that region of the sleeve which is first shrunk onto the cable insulator at the portion of the support to be removed initially. In fact, that region of the sleeve would be shrunk onto the underlying cable, at its positioning, with such force that it can be moved no more.
It has been found, therefore, that the sleeve positioning can be significantly improved by having a selected internal region, preferably the middle region, shrunk first, rather than a peripheral region of the splice.